Bandmill with automatic track and strain control system

ABSTRACT

A bandmill has first and second wheels and an endless saw blade trained about the wheels. The wheels are mounted to rotate about respective axes which lie in a common plane, but the first wheel is adjustable in position relative to the second wheel with respect to both its distance from the second wheel and its orientation relative to the second wheel. A tensioning mechanism is effective to force the first wheel away from the second wheel, while maintaining the axes of rotation of the two wheels substantially parallel, in response to a signal which is provided by a load cell and represents the tension in the saw blade. A wheel tilt mechanism is effective to adjust the angular position of the first wheel relative to the second wheel in response to a signal indicating the path followed by the saw blade, for maintaining the blade on a proper path.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 07/151,803 filed Feb. 3,1988 and now abandoned, which is a continuation-in-part of co-pendingapplication Ser. No. 07/022,096 filed Mar. 5, 1987, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a bandmill having an automatic track andstrain control system.

A bandmill is used for cutting lumber. A conventional vertical bandmillcomprises a support frame, two wheels, one disposed vertically above theother, and an endless saw blade trained about the wheels. The lowerwheel is driven and the upper wheel idles. The blade is maintained undertension, and accordingly when the lower wheel is driven the blade passesendlessly about the wheels. A cutting throat is provided between theupper and lower wheels along the downward run of the blade. In order toensure that the blade remains in position on the wheels, the wheels arecrowned and their relative positions are accurately determined so thatthe plane containing the maximum circumference of the upper wheelcoincides with the plane containing the maximum circumference of thelower wheel. This need for accurate positioning of the wheels impliesthat the conventional bandmill is expensive to construct because amassive support structure is required in order to support the wheelswith the required degree of stability.

The support structure that is conventionally used for a bandmillcomprises a concrete base and a support frame mounted on the base. Thesupport frame includes a mechanism for adjusting the vertical positionof the upper wheel, whereby the tension in the blade can be adjusted.However, if the upper wheel is moved, it is then necessary to readjustthe relative positions of the wheels to achieve precise coincidence ofthe planes containing the maximum circumference of the two wheels.

Sometimes, it is desirable to employ a bandmill in which the bladepasses through the cutting throat in the horizontal direction, or at anangle that is inclined to the horizontal. However, the support structureof the vertical bandmill does not permit ready adjustment of theorientation of the bandmill. The nature of the support structure alsoimplies that the orientation in which a given bandmill will be used isfixed at the time of manufacture, and accordingly it is necessary tobuild distinct bandmills for vertical, horizontal and inclined use.

The support structure for the conventional bandmill is not only massivebut is also bulky. If two conventional bandmills are disposed in thesame orientation and on the same side of the cutting path, they can notbe any closer together than about two feet. It is proposed in co-pendingapplication Ser. No. 07/089,489 filed Aug. 21, 1987, that a sawmillshould be provided in which any three bandmills out of a group of fourbandmills act on a single log on a single pass of the log through abandmill station. The four bandmills are stationary with respect to thedirection of feed of the log through the group of bandmills. The sawmillis designed to process logs that are only eight feet long. The cuttingedges of the saw blades must therefore be closer together than about twofeet, since otherwise the sawing by the upstream bandmill will becompleted before the log starts to be sawn by the downstream bandmill,and this may create difficulties in log handling.

During sawing, the saw blade of a conventional bandmill vibrates in thecutting throat. If the blade is dull or the feed rate is high, the bladetends to snake out of the desired cutting plane as a log is fed throughthe throat. Consequently, lumber with rough surfaces or of uneventhickness is produced. Moreover, snaking may result in the blade beingdeflected to such an extent that it strikes other parts of the bandmill,resulting in damage to the bandmill. Hitherto, these problems have beenaddressed by using a thicker blade, which results in greater kerf loss,or increasing the blade tension, which results in higher maintenancecosts.

When the blade of a bandmill is deflected, e.g., due to nonuniformitiesin the log being cut, there is a tendency for blade oscillations tooccur, resulting in thicker kerf and impaired accuracy.

The stress distribution in the blade of a bandmill depends on the pathfollowed by the blade relative to the wheels, and the stressdistribution has a bearing on sawing accuracy. It has been found thatimproved accuracy is obtained if the bottoms of the gullets of the teethare maintained close to the edges of the wheels, so that the stiffnessof the blade in the teeth area is maximized. However, if the gulletspass over the wheels, there is a tendency for the saw blade to crack inthe vicinity of the gullets.

SUMMARY OF THE INVENTION

A preferred embodiment of the present invention in its first aspect is abandmill having at least first and second wheels and an endless sawblade trained about the wheels. The wheels are mounted to rotate aboutrespective axes which lie in a common plane, but the first wheel isadjustable in position relative to the second wheel with respect to bothits distance from the second wheel and its orientation relative to thesecond wheel. A tensioning mechanism is effective to force the firstwheel away from the second wheel, while maintaining the axes of rotationof the two wheels substantially parallel, in response to a signal whichis provided by a load cell and represents the tension in the saw blade.A wheel tilt mechanism is effective to adjust the angular position ofthe first wheel relative to the second wheel in response to a signalindicating the path followed by the saw blade, for maintaining the bladeon a proper path.

By providing for adjustment of the tilt of the first wheel, it ispossible to avoid the need for the massive support structures that areused in conventional bandmills, since the tilt mechanism maintains thesaw blade on the proper path. The tensioning mechanism enables a highblade tension to be achieved, so that the amplitude of blade vibrationis reduced, and this results in consistent sawing. However, the tensionis controlled so that it remains low enough that unacceptablemaintenance costs are avoided.

A preferred embodiment of the present invention in a second aspect isbandmill apparatus comprising at least first and second wheel arborseach having first and second opposite ends and each defining a centralaxis, and a support structure which supports the first and second wheelarbors with their central axes lying in a common plane. The supportstructure comprises two columns which are held in stationary,spaced-apart relationship and each of which receives one end of each ofthe wheel arbors. First and second wheels are mounted on the first andsecond arbors respectively, in the space between the two columns, andare rotatable with respect to the support structure about the centralaxes of the arbors respectively.

Use of such a support structure ensures that when the support structureis moved, the wheel arbors remain in the same relative positions unlessone or both of the wheel arbors is moved relative to the supportstructure, and this in turn permits the orientation of the bandmillapparatus to be changed, e.g., from a vertical orientation to ahorizontal orientation, without its being necessary to dismantle andrebuild the entire bandmill apparatus. This in turn implies that a givenbandmill can be installed in any orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings in which:

FIG. 1 is a side elevation of a horizontal two-wheel bandmill embodyingthe present invention,

FIG. 2 is a sectional view to an enlarged scale showing the manner ofmounting of the driven wheel of the FIG. 1 bandmill,

FIG. 3 is a vertical sectional view to an enlarged scale showing themanner of mounting of the idler wheel of the FIG. 1 bandmill,

FIG. 4 is a side elevation, partly cut away, showing the idler wheel ofthe FIG. 1 bandmill and the tilt and strain mechanisms therefor,

FIG. 5 is a partial vertical sectional view of the FIG. 1 bandmillillustrating the idler wheel and the tilt and strain mechanism,

FIG. 6 illustrates part of the blade of the FIG. 1 bandmill and showsfeatures relevant to the positioning of sensor units,

FIG. 7 illustrates the arrangement of sensor units relative to theblade,

FIG. 8 illustrates the control circuit for the tilt and strainmechanisms,

FIG. 9 illustrates waveforms used in explaining the operation of thetilt mechanisms,

FIG. 10 illustrates a modification of FIG. 3,

FIG. 11 is a simplified side view of a threewheel bandmill embodying thepresent invention, and

FIG. 12 is a sectional view of the FIG. 11 bandmill, taken on the lineXII--XII of FIG. 11.

The FIG. 1 bandmill is illustrated in the

drawings in a vertical orientation. However, it is disposed horizontallywhen in use.

In the different figures of the drawings, like reference numerals denotecorresponding elements. In FIGS. 11 and 12, primed reference numeralsdenote elements having similar functions to the elements denoted by thecorresponding unprimed numerals in FIG. 9.

DETAILED DESCRIPTION

The bandmill illustrated in FIGS. 1-8 comprises a support frame 4, adriven wheel 6, an idler wheel 8, and an endless saw blade 66, which isshown in FIGS. 6 and 7 but not in FIGS. 1-5. The support frame 4comprises two rigid steel columns 4A and 4B which are held in spacedapart, parallel relationship, e.g. by plates 4C that are welded to thecolumns 4A, 4B. As shown in FIG. 2, the driven wheel 6 is keyed to anarbor 10 which is journalled in bearings 14. The bearings 14 are mountedin the columns 4A, 4B respectively. The wheel 6 is coupled by way of thearbor 10 to a hydraulic drive motor 17, which receives hydraulic fluidunder pressure from a pump 32 (FIG. 8) by way of a valve 34, which iscontrolled by a solenoid 134. The valve 34 has a first position in whichit delivers fluid under pressure from the pump 32 to the motor 17, fordriving the motor, and a second position in which it prevents deliveryof fluid to the motor. The coupling between the motor 17 and the arbor10 is through sheaves and belts, but it may alternatively be a directdrive coupling. The idler wheel 8 (FIG. 3) is journalled on an arbor 12by means of bearings 16. The arbor 12 is supported at its two oppositeends in spherical bearings 50 which are themselves supported inrespective carriages 52. The carriages 52 are slidable along rods 54which are secured rigidly to the support frame 4, bearings 53 beingdisposed between the carriages 52 and the rods 54.

A differential lever assembly comprising two cranked levers 22A and 22B(FIGS. 1, 4, 5) is mounted on a shaft 24 of the support frame 4 by wayof sleeve bearings. The lever assembly is accommodated between the twocolumns 4A, 4B. Each lever 22 has two arms 40 and 42. At their endsfarther from the shaft 24, the arms 40 carry rollers which engage wearplates on the carriages 50 respectively. At its end farther from theshaft 24, the arm 42A is coupled to the support frame by way of asingle-acting hydraulic cylinder 28 and is coupled to the arm 42B by wayof a double-acting hydraulic cylinder 30. As shown in FIG. 8, thecylinders 28 and 30 are connected to the pump 32 by way of respectivevalves 36 and 38. The valve 36 is a servo valve that is controlled by asolenoid 136 and delivers fluid under pressure to the cylinder 28 toextend the cylinder, or allows hydraulic fluid to leave the cylinder, sothat the cylinder can retract, at a rate dependent on the magnitude ofthe current received by the solenoid 136. The valve 38 is controlled bytwo solenoids 138b and 138f and has a first position in which itdelivers fluid under pressure to one chamber of the cylinder 30 andallows fluid to leave the other chamber of the cylinder 30, a secondposition in which it allows fluid to leave the one chamber and deliversfluid under pressure to the other chamber, and a third position in whichfluid does not enter or leave either chamber except through leakage.

Upon delivery of fluid under pressure to the cylinder 28, the leverassembly is urged to pivot about the axis of the shaft 24 in theclockwise direction shown in FIG. 1, with the relative angular positionsof the arms 42 depending on the condition of the valve 38. The carriages50 are forced away from the wheel 6 (to the left of FIG. 1) and thewheel 8 also is forced away from the wheel 6. In this manner, the sawblade is placed under tension. The differential nature of the leverassembly allows the arbor 12, and the Wheel 8 carried thereby, to betilted to a selected extent relative to the arbor 10 and the wheel 6.

A load cell 70 is interposed between the cylinder 28 and the arm 42A andprovides a pressure signal representative of the force exerted by thecylinder 28 on the lever 22A. The tension in the saw blade 12 dependsupon the force exerted by the cylinder 28, and the pressure signalprovided by the load cell 70 is applied to a programmable logiccontroller (PLC) 72 (FIG. 8). The PLC 72 compares the tension valueindicated by the pressure signal with a pre-set range of values. If thetension value indicated by the pressure signal is outside the pre-setrange, the PLC provides a current to the solenoid 136 of appropriatemagnitude and direction, using an analog PID control loop, to restorethe indicated tension value to the pre-set range of values. In thismanner, the tension in the saw blade is maintained substantiallyconstant. In the event that the pressure signal starts to vary inoscillating fashion, implying that the blade is oscillating, thecontroller 72 adjusts the valve 36 so as to damp the oscillation withoutreducing the blade tension substantially.

FIG. 6 illustrates a portion of the saw blade 66 in plan, at a locationin the cutting throat. It will be seen from FIG. 6 that each tooth 61 isgenerally triangular in form and that adjacent teeth are separated bygullets 63.

The plane of the forward edge of the driven wheel 6 (the upstream edgewith respect to the direction of lumber flow) intersects the planedefining the nominal position of the blade 66 in the cutting throat in aline 68. For proper operation of the bandmill, the line 62 that definesthe base of the gullets must lie just forward of the line 68. It isdesirable that the line 62 be 1/16" +/-1/16" forward of the line 68.

Three sensor units 64b, 64f and 64c are mounted above the plane of thelower run of the blade 66, just upstream (with respect to the directionof movement of the blade) of the cutting throat 60. The sensor units 64are used to sense the presence of the blade or its teeth atpredetermined locations. It is known to use inductive sensors to sensethe presence of a metal object, such as a saw blade, but the short rangeof conventional inductive sensors renders them less than optimal fordetecting whether the blade of a bandmill is at a desired position.Thus, the blade is likely to depart from its nominal path by a distancegreater than the range of an inductive sensor, resulting in apossibility that the blade will strike the sensor and destroy it.Therefore, use of photoelectric sensor units is preferred. In thepreferred embodiment of the invention, each sensor unit is of the kindmanufactured by Banner Engineering Corp. and sold under the designationSBCI-6. The sensor unit includes a light source (a light-emitting diode)and a photodetector (a photodiode). As shown in FIG. 7, each sensor unit64 emits a light beam along an optical axis 104. The light beam isdirected towards the saw blade. Light incident on the saw blade createsa reflected beam which is collected by the sensor unit 64 and isdirected onto the photodetector. The LED is imaged onto thephotodetector if the saw blade is at a distance of from 5 to 7 inchesfrom the sensor unit. The sensor unit provides a 24 volt d.c. outputsignal if the LED remains imaged on the photodetector for a time longerthan the response time of the sensor unit, and otherwise its output isat 0 volts. The output signals provided by the sensor units are appliedto the PLC 72. In order to immunize the sensor unit from the influencesof ambient light, the light beam provided by the sensor unit ismodulated in intensity at 10 kHz and the signal provided by thephotodetector is demodulated against a reference signal at 10 kHz.Demodulation occurs over 10 cycles of the reference signal, andaccordingly the response time of the sensor unit is 1 ms.

The optical axis 104b of the sensor unit 64b is vertical and intersectsthe plane of the blade at a distance D1 from the line 68. The opticalaxis 104f of the sensor unit 64f is vertical and intersects the plane ofthe blade at a distance D2 from the line 68. When the blade is in theideal position relative to the wheel 6, the line 64 that defines theposition on the teeth where the length of the gullet is equal to halfthe distance between the points of the teeth is at a distance D from theline 68, and the distance D1 is equal to D plus 1/16" whereas thedistance D2 is equal to D minus 1/16". Therefore, when the blade iswithin the proper tracking range, the light intensity received by eachof the sensor unit 64b and 64f varies substantially in accordance with arectangular waveform, with the duty cycle of the intensity for thesensor 64b somewhat less than 50% and the duty cycle of the intensitywaveform for the sensor 64f somewhat more than 50%.

In an embodiment of the invention, the saw blade is driven at a speed of10,000 ft/min (50.80 m/s) and the teeth are at a pitch of 1.75 in (4.445cm). Accordingly, the frequency at which the light intensity received bythe sensor units 64b, 64f varies at normal operating speed is about 1143Hz, corresponding to a period of 0.875 ms. However, when the bandmill isstarted, the frequency at which the light intensity received by thesensor units 64b, 64f varies is much less than 1143 Hz, and thefrequency increases as the blade is accelerated. FIG. 9 illustrates fourpairs of waveforms A, B, C, and D, in which the upper waveform of eachpair represents the variation in light intensity received by the unit64b at a given blade speed and the lower waveform represents thecorresponding variation in the output voltage provided by the unit 64b.

The PLC 72 includes digital filters 72b, 72f which receive the outputsignals provided by the sensor units 64b, 64f and provide bi-leveloutput signals for analysis by the PLC. The output signal provided bythe filter 72b or 72f is high if the duty cycle of the intensitywaveform received by the appropriate sensor unit is greater than 50% andlow if the duty cycle is less than 50%. During start-up of the bandmill,the digital filters are adjusted to take account of the progressiveincrease in the frequency of the intensity waveforms. Adjustment of thefilters is performed automatically as a function of time, becausevariations in the rate at which the speed of the blade increases duringstart-up are quite small.

When the path of the blade shifts forwards from the ideal position bymore than 1/16", the duty cycle of the intensity waveform for the sensorunit 64b becomes greater than 50% and therefore the output voltageprovided by the filter 72b will go high. Conversely, when the pathshifts backwards from the ideal position by more than 1/16", the dutycycle of the intensity waveform for the unit 64f becomes less than 50%and the output signal provided by the filter 72f will go low. Thesignals provided by the filters 72b and 72f therefore indicate whetherthe path along which the saw blade is passing is within the properrange. If the blade is displaced vertically by more than about 1 inchfrom its ideal position, the intensity of reflected light received bythe sensor units 64 is not sufficient to cause the detector to respondand accordingly the output voltages provided by the sensor units go low.

During operation of the bandmill, the PLC functions both in a controlmode and in a diagnostic mode. In the control mode, action is taken bythe PLC on the basis of the output provided by a single sensor unit 64for 64b. In particular, if the output voltage of the filter 72b goeshigh, indicating that the blade is forward of the proper tracking range,the PLC provides an output signal to energize the solenoid 138bmomentarily. Energization of the solenoid 138b causes the valve 38 todeliver fluid to the double-acting cylinder 30 in the sense to force thebackward end of the arbor 12 away from the arbor 10 relative to theforward end of the arbor 12. This change in the orientation of the idlerwheel tends to shift the blade backwards. Conversely, if the outputvoltage provided by the filter 72f goes low, the solenoid 138f isenergized and the resulting change in orientation of the idler wheeltends to shift the blade forwards. The delay time through the sensors,the PLC and valve drive solenoids 138f, 138 b, between the bladeshifting out of the proper tracking range and the valve 38 beingadjusted in response thereto, is about 50 ms. In order to preventovercompensation, the controller 72 samples the outputs of the filters72b, 72f at intervals longer than 50 ms, so that the effect of eachadjustment of the valve 38 on the output signals of the filters 72b, 72fcan be observed before another adjustment is made.

The third sensor unit 64c is positioned to illuminate, and receive lightreflected from, a region of the blade that is in the center of the bladewith respect to the direction of lumber flow. Normally, the outputsignal provided by the unit 64c is high. In the diagnostic mode, the PLCdiagnoses hardware faults in the tracking control system. In particular,if the output signals of the filters 72b and 72f are high and the outputsignal of the sensor unit 64c is low, an indication is given that thesensor unit 64c is faulty; and if the output signals of the filter 72band the sensor unit 64c are high but that of the filter 72f is low, anindication is given that the sensor unit 64b is faulty. Also, if thetracking of the blade is being adjusted, by energizing one of thesolenoids 138b, 138f, and adjustment continues over more than 20 secondswithout the blade being detected as having been restored to the propertracking range, an indication of a fault in the double-acting solenoidvalve is given.

The PLC is also able to diagnose faulty conditions in the sawingoperation: if the output signals of the filter 72f and the sensor unit64c both go low while the output signal of the load cell remains at anacceptable level, an indication that the blade is snaking is given, andthe log feed is slowed down or stopped. If slowing or stopping the logfeed does not result in the output signal of the filter 72f and thesensor unit 64c going high, the motor 17 is stopped. If the outputsignal of the sensor unit 64c goes low and the output signal of the loadcell indicates that the tension in the blade is too low, an indicationthat the blade is broken is given, and the motor 17 is stopped.

Switches 139f and 139b are connected to the solenoids 138f and 138b toallow manual adjustment of the valve 38. Similarly, a switch 135 isprovided to allow the motor to be stopped under operator control.

FIG. 10 shows a modification of FIG. 3, according to which the upperwheel 8 is keyed to the arbor 12. The opposite ends of the arbor arejournalled in bearings 78 which are supported by the carriages 52. Thebearings 78 permit both tilting and rotation of the arbor 12.

The bandmill shown in FIGS. 11 and 12 is a three-wheel bandmill having adriven wheel 6 and two idler wheels 8 and 80. The idler wheel 8 is keyedto the arbor 12, and the arbor 12 is mounted in the support frame 4 bymeans of bearings (not shown) which are stationary relative to thebearings of the arbor 10. The third wheel is supported by bearings on anarbor 82 which is mounted in the support frame 4 by way of a leverassembly comprising levers 22A' and 22B'. The levers 22' pivot about thecentral axis of a shaft 24', and each lever has two arms 40' and 42'. Attheir ends farther from the shaft 24', the arms 40' carry respectivespherical bearings 84. The two spherical bearings 84 receive theopposite ends of the arbor 82. At its end farther from the shaft 24', ofa hydraulic cylinder 28 and is coupled to the arm 42B' by way of ahydraulic cylinder 30.

The bandmills illustrated in FIGS. 1-10 are preferred over that of FIGS.11 and 12 because the wheel 80 of the FIGS. 11 and 12 bandmill is muchsmaller in diameter than the wheels 6 and 8 and therefore the bladeundergoes more severe bending stresses when passing around the wheel 80than when passing around the wheels 6 and 8.

It will be appreciated that the present invention is not restricted tothe particular embodiments that have been described and illustrated, andthat variations may be made therein without departing from the scope ofthe invention as defined in the appended claims and equivalents thereof.For example, it is not essential to the invention in its broadestaspects that the tilting of the wheel 8 or 80 take place by applyingforces in the same direction as the forces used to tension the sawblade. Other methods of sensing the path followed by the saw blade thanuse of photodetectors in the manner described above may be used. Theinvention is not restricted to the load cell being in the position shownin the drawings. Two load cells may be provided between the arbor 12 andthe lever assemblies 40a and 40b respectively.

We claim:
 1. A bandmill comprising a support frame, a first wheel, anarbor having two opposite ends and on which the first wheel is mountedfor rotation about an axis thereof, two carriages in which the twoopposite ends of the arbor are received respectively and which aremovable relative to the support frame, a second wheel mounted in thesupport frame for rotation about an axis of the second wheel, an endlesssaw blade trained about the wheels, a differential lever assembly havingfirst and second levers which are mounted pivotally to the support frameand engage the two carriages respectively, tensioning means for forcingthe first wheel away from the second wheel so as to place the saw bladeunder tension, tension control means for measuring the tension in theblade and acting automatically upon the tensioning means to maintain thetension in the blade within a predetermined range, tilt means foradjusting the angular position of the axis of rotation of the firstwheel relative to the axis of rotation of the second wheel, and tiltcontrol means for sensing the path along which the blade passes andacting automatically upon the tilt means for maintaining the path of theblade within a predetermined range, and wherein the tensioning means andthe tilt means comprise a first force member which is effective betweenthe support frame and the first lever and a second force member which iseffective between the second lever and the first lever.
 2. A bandmillaccording to claim 1, wherein the tensioning control means comprise aforce sensor which is effective to measure the value of the forceapplied to the first lever by the first force member, and a controllerwhich receives the measured value of the force applied to the firstlever by the first force member and controls the first force member tomaintain the measured value substantially equal to a predeterminedvalue.
 3. A bandmill according to claim 2, wherein the force sensorcomprises a load cell disposed between the first force member and thefirst lever.
 4. A bandmill according to claim 1, wherein the tiltcontrol means comprise sensor means for sensing the path along which theblade passes, and a controller which determines whether the sensed pathsubstantially coincides with the predetermined path and controls thesecond force member to maintain the sensed path in substantialcoincidence with the predetermined path.
 5. A bandmill according toclaim 4, wherein the sensor means comprise first and secondphotodetectors for collecting light reflected from the saw blade andgenerating electrical signals representative of the power at which lightis received by the photodetectors respectively, the first photodetectorbeing positioned to collect light reflected from the teeth of the bladewhen the blade is passing along the proper path and the secondphotodetector being positioned to collect light reflected from the bodyof the blade when the blade is passing along the proper path.
 6. Abandmill according to claim 4, wherein the sensor means comprise firstand second sensor units, which are stationary with respect to thesupport frame, for collecting light reflected from the saw blade andgenerating electrical signals representative of the duty cycle at whichreflected light is collected by the sensor units respectively as the sawblade is driven along its path of movement, whereby the sensor unitsscan the saw blade, the sensor units being positioned to scan the sawblade along paths that cross the teeth of the blade at different levelsof the teeth between the gullets and the tips of the teeth, whereby theduty cycle at which reflected light is collected by each sensor unitdepends on the position of the blade relative to the predetermined path.7. A bandmill according to claim 6, wherein the positions of the firstand second sensor units are selected such that when the blade is on thepredetermined path, the duty cycle at which reflected light is collectedby the first sensor unit is less than 50% and the duty cycle at whichreflected light is collected by the second sensor unit is greater than50%, and the controller responds to the first sensor unit collectingreflected light at a duty cycle of more than 50% or the second sensorunit collecting reflected light at a duty cycle of less than 50% bycontrolling the second force member to restore the blade to thepredetermined path.
 8. A bandmill according to claim 7, furthercomprising a third sensor unit, which is stationary with respect to thesupport frame, for collecting light reflected from the saw blade andgenerating an electrical signal representative of the power at whichreflected light is collected by the third sensor unit as the blade isdriven along its path of movement, whereby the third sensor unit scansthe saw blade, the third sensor unit being positioned to scan the sawblade along a path that is substantially equidistant from the back ofthe blade and the gullets of the teeth of the blade, and wherein thecontroller is programmed to provide a fault indication if the controllerreceives a signal from the third sensor unit indicating that it iscollecting reflected light at a predetermined power and receives asignal from the first sensor unit indicating that it is collecting lightat a duty cycle of more than 50% but the controller does not receive asignal from the second sensor unit indicating that it is collectinglight at a duty cycle of more than 50%.
 9. A bandmill according to claim6, further comprising a third sensor unit, which is stationary withrespect to the support frame, for collecting light reflected from thesaw blade and generating an electrical signal representative of thepower at which reflected light is collected by the third sensor unit asthe blade is driven along its path of movement, whereby the third sensorunit scans the saw blade, the third sensor unit being positioned to scanthe saw blade along a path that is substantially equidistant from theback of the blade and the gullets of the teeth of the blade.
 10. Abandmill according to claim 9, wherein the third sensor unit isconnected to the controller and the controller is programmed to providea fault indication if the output signals provided by the first andsecond sensor units indicate that they are collecting light reflectedfrom the teeth of the blade but the controller does not receive a signalindicating that the third sensor unit is collecting reflected light at apredetermined power level.
 11. A bandmill according to claim 9, whereinthe tension control means comprise a force sensor which is effective tomeasure the value of the force applied to the first lever by the firstforce member, and the controller receives the measured value of theforce applied to the first lever by the first force member and controlsthe first force member to maintain the measured value substantiallyequal to a higher predetermined value, and the controller is programmedto provide a fault indication if the measured value of the force appliedto the first lever by the first force member falls to a lowerpredetermined value and the controller does not receive a signal fromthe third sensor unit indicating that it is collecting reflected lightat a predetermined power.
 12. A bandmill according to claim 1, whereinthe first and second force members are hydraulic cylinders.
 13. Abandmill according to claim 1, wherein the support frame comprises twocolumn members and means connecting the column members and maintainingthem in spaced, substantially parallel relationship, and the first andsecond wheels are disposed in the space between the two column members.14. A bandmill according to claim 13, wherein the two carriages aremounted to the two column members respectively and are movableindependently of each other relative to the support frame, whereby theangular position of the first wheel relative to the support frame isadjustable, and the bandmill further comprises a second arbor having twoopposite ends and on which the second wheel is mounted, the two oppositeends of the second arbor being mounted to the two column membersrespectively in a manner preventing movement of the second arborrelative to the support frame in directions perpendicular to said axisof rotation of the second wheel.
 15. A bandmill according to claim 13,wherein the first and second levers are disposed in the space betweenthe two column members.
 16. A bandmill according to claim 15, whereineach lever has first and second arms and the first arms of the twolevers engage the two carriages respectively, and the first force memberis effective between the support frame and the second arm of the firstlever and the second force member is effective between the second arm ofthe second lever and the second arm of the first lever.
 17. A bandmillaccording to claim 1, wherein each lever has a first arm and a secondarm, the first arms of the two levers engaging the two carriagesrespectively, and the first force member is effective between thesupport frame and the second arm of the first lever and the second forcemember is effective between the second arm of the second lever and thesecond arm of the first lever.
 18. A bandmill having first, second andthird wheels, means supporting the wheels for rotating about respectiveaxes, and endless saw blade trained about the wheels, tensioning meanseffective to force the first wheel away from the second wheel so as toplace the saw blade under tension, tension control means for measuringthe tension in the blade and acting upon the tensioning means tomaintain the tension at a predetermined value, tilt means for adjustingthe angular position of the axis of rotation of the first wheel relativeto the axis of rotation of at least one other wheel, and tilt controlmeans for sensing the path along which the blade passes and acting uponthe tilt means for maintaining the blade on a predetermined path.
 19. Abandmill according to claim 18, wherein the second and third wheels areof substantially the same diameter and the first wheel is of lesserdiameter than the second and third wheels.
 20. A bandmill according toclaim 18, comprising a support frame in which the second and thirdwheels are mounted, an arbor having two opposite ends and on which thefirst wheel is mounted, and a lever assembly having two levers by whichthe two opposite ends of the arbor are supported respectively, whereinthe two levers are mounted to pivot about a common axis which isparallel to the axes of rotation of the second and third wheels and isstationary with respect thereto, and wherein the tensioning means andthe tilt means comprise a first force member which is effective betweenthe support frame and one of the two levers and a second force memberwhich is effective between said one lever and the other lever.
 21. Abandmill comprising a support frame, a first wheel, an arbor having twoopposite ends and on which the first wheel is mounted for rotation aboutan axis thereof, first and second arbor supports which the two oppositeends of the arbor are received respectively and which are movablerelative to the support frame, a first link member effective on thefirst arbor support, a second link member effective on the second arborsupport, a second wheel mounted in the support frame for rotation aboutan axis of the second wheel, an endless saw blade trained about thewheels, tensioning means effective to force the first wheel away fromthe second wheel so as to place the saw blade under tension, tensioncontrol means for measuring the tension in the blade and actingautomatically upon the tensioning means to maintain the tension within apredetermined range, tilt means for adjusting the angular position ofthe axis of rotation of the first wheel relative to the axis of rotationof the second wheel, and tilt control means for sensing the path alongwhich the blade passes and acting automatically upon the tilt means formaintaining the path of the blade within a predetermined range, andwherein the tensioning means comprise first force means effectivebetween the support frame and the first link member and the tilt meanscomprise second force means effective between the first link member andthe second link member.
 22. A bandmill according to claim 21, whereinthe tension control means comprise a transducer that provides a signaldependent on the force applied to the first link member by the firstforce means, and a controller responsive to the transducer forcontrolling the first force means.
 23. A bandmill comprising a supportframe, a first wheel, a first arbor having two opposite ends and onwhich the first wheel is mounted for rotation about an axis thereof, twocarriages in which the two opposite ends of the first arbor are receivedrespectively and which are movable relative to the support frame, asecond wheel mounted in the support frame for rotation about an axis ofthe second wheel, an endless saw blade trained about the wheels,tensioning means effective to force the first wheel away from the secondwheel so as to place the saw blade under tension, tension control meansfor measuring the tension in the blade and acting automatically upon thetensioning means to maintain the tension within a predetermined range,tilt means for adjusting the angular position of the axis of rotation ofthe first wheel relative to the axis of rotation of the second wheel,and tilt control means for sensing the path along which the blade passesand acting automatically upon the tilt means for maintaining the path ofthe blade within a predetermined range, and wherein the support framecomprises first and second frame members between which the first wheeland the second wheel are disposed and on which the two carriages arerespectively mounted, the bandmill also comprising a second arbor onwhich the second wheel is mounted, the second arbor being supported bythe first and second frame members at locations spaced apart along thesecond arbor.
 24. A bandmill according to claim 23, wherein the secondwheel is keyed to the second arbor, and the second arbor is journaled infirst and second bearings received by the first and second frame membersrespectively.
 25. A bandmill comprising a support frame, a first wheel,an arbor having two opposite ends and on which the first wheel ismounted for rotation about an axis thereof, two carriages in which thetwo opposite ends of the arbor are received respectively and which aremovable relative to the support frame, a second wheel mounted in thesupport frame for rotation about an axis of the second wheel, an endlesssaw blade trained about the wheels, tensioning means effective to forcethe first wheel away from the second wheel so as to place the saw bladeunder tension, tension control means for measuring the tension in theblade and acting automatically upon the tensioning means to maintain thetension within a predetermined range, tilt means for adjusting theangular position of the axis of rotation of the first wheel relative tothe axis of rotation of the second wheel, and tilt control means forsensing the path along which the blade passes and acting automaticallyupon the tilt means for maintaining the path of the blade within apredetermined range, and also comprising bearing means whereby the firstwheel is supported for rotation about the arbor, and first and secondspherical bearings whereby said two opposite ends of the arbor aresupported by the carriages respectively.
 26. A bandmill comprising asupport frame, a first wheel, an arbor having two opposite ends and onwhich the first wheel is mounted for rotation about an axis thereof, twocarriages in which the two opposite ends of the arbor are receivedrespectively and which are movable relative to the support frame, asecond wheel mounted in the support frame for rotation about an axis ofthe second wheel, an endless saw blade trained about the wheels, adifferential lever assembly having first and second levers which aremounted pivotally to the support frame, the two levers engaging the twocarriages respectively, tensioning means effective to force the firstwheel away from the second wheel so as to place the saw blade undertension, tension control means for measuring the tension in the bladeand acting automatically upon the tensioning means to maintain thetension within a predetermined range, tilt means for adjusting theangular position of the axis of rotation of one of the wheels relativeto the axis of rotation of the other wheel, and tilt control means forsensing the path along which the blade passes and acting automaticallyupon the tilt means for maintaining the path of the blade within apredetermined range, and wherein the tensioning means and the tilt meanscomprise a first force member effective between the support frame andthe first lever and a second force member effective between the secondlever and the first lever.